Most unobtrusive: simple delays. EQs on both send and return help A LOT.
Next in line: early reflections.
Always welcome: well-arranged tunes and mic placement.
I'm moving further and further away from reverbs these days.
"Recording exactly what a musician hears turns out to be a really big deal." Bob Olhsson
"Who cares about efficiency, when we're talking about music?" Rupert Neve
"it'll sound different through a microphone, anyway" Keith Carlock
"no room, no boom!" Michael Wagener